The present invention is a process for treating a by-product stream, resulting from the commercial manufacture of methylchlorosilanes, to remove residual silanes where the residual silanes comprise low-boiling hydrosilanes. The process comprises a first step where the by-product stream is contacted with hydrogen chloride in the presence of a chlorination catalyst to convert the low-boiling hydrosilanes to chlorosilanes. In a second step, the modified by-product stream comprising the chlorosilanes is contacted with activated carbon which adsorbs higher-boiling silanes including chlorosilanes, thereby forming a final by-product stream reduced in lower-boiling hydrosilanes and in total silanes. The present process is especially useful for removing silanes from by-product streams intended for incineration, thus reducing silica during combustion of the by-product stream in the incinerator.
The commercial production of methylchlorosilanes involves the contact of methyl chloride with silicon metalloid in the presence of a catalyst comprising copper at temperatures generally within a range of about 300.degree. C. to 350.degree. C. Typically this process is optimized for the production of dimethyldichlorosilane, with lessor amounts of methylsilanes, methylchlorosilanes, methylhydrosilanes, C.sub.2 to C.sub.5 hydrocarbons, polysilanes, polysiloxanes, silylmethylenes, and other species being formed. The product mixture can also contain unreacted methyl chloride. This product mixture usually undergoes a series of process steps such as distillation, rearrangement, condensation, and the like, to optimize the recovery of dimethyldichlorosilanes or other desired silanes. A consequence of these production processes is by-product streams. Such by-product streams can result, for example, from the venting or purging of equipment used in making and isolating the methylchlorosilanes. The by-product streams can be highly concentrated in by-products or may be diluted in inert purge gases such as nitrogen.
The present process provides a method where essentially all of the silanes present in such by-product streams can be recovered from the by-product streams. The silanes can then be further processed into useful products. The present process is especially useful for treating by-product streams which are to be incinerated to make them safe for venting to the environment. During the incineration process silanes in the by-product stream can pyrolyze to form particulate silica which can collect in the incinerator creating heat transfer problems and causing plugging. Removal of silanes from the by-product stream by the present process can greatly reduce the formation of particulate silica in an incinerator.
The present inventors have found that activated carbon can adsorb silanes, including chlorosilanes, methylchlorosilanes, methylsilanes, and methylhydrochlorosilanes, from a by-product stream. However, activated carbon is not effective for adsorbing the lower-boiling hydrosilanes such as methylsilane, dimethylsilane, and silane (SiH.sub.4), from such by-product streams. Provided herein is a process where the by-product stream is first contacted with hydrogen chloride in the presence of a chlorination catalyst, to chlorinate the low-boiling hydrosilanes converting them to chlorosilanes which are adsorbed by activated carbon. By this process essentially all of the silanes can be recovered from the by-product stream.
Petrov et al., Synthesis of Organosilicon Monomers, Consultants Bureau, NY, N.Y., 1964, p. 416, report that the Si--H bonds of silicon hydrides such as SiH.sub.4, ClSiH.sub.3, and CH.sub.3 SiH.sub.3 will react with hydrogen chloride in the presence of AlCl.sub.3 to effect chlorination of the silicon hydride.
Sommer et al., J. Org. Chem. 32:2470-2472 (1967), disclose that organosilicon hydrides react with hydrogen halides in the presence of Group VIII metals to form organosilicon halides and halogen. Sommer et al. studied the reaction of (C.sub.2 H.sub.5).sub.3 SiH with HCl to produce (C.sub.2 H.sub.5).sub.3 SiCl.
Bokerman et al., U.S. Pat. No. 4,985,579 describe a process for the removal of hydrogen containing silane impurities from organosilanes with similar boiling points. In the process described by Bokerman et al., the hydrogen containing silane is reacted with a hydrogen halide in the presence of a halogenation catalyst to replace the hydrogen on the silane with a halide and create a modified silane. The substitution of the heavier halide for the hydrogen increases the boiling point of the modified silane to facilitate the separation of the modified silane from organosilanes having a boiling point similar to that of the modified silane prior to halogenation.
Zizka et al., Czech. Pat. No. 89396, published Apr. 15, 1959, describe a process for the recovery of methyl chloride from a gaseous by-product stream containing silanes. Zizka et al. teach treating the by-product stream with dilute NaOH or KOH solution to remove hydrolyzable silanes. The treated gas is then contacted with activated carbon, which adsorbs the methyl chloride. The methyl chloride is recovered from the activated carbon by thermal desorption.